1. Field of the Invention
The present invention relates to the use of fluids having perfluoropolyether structure as testing media in the electronic industry.
More particularly, the invention relates to the use of such perfluoropolyethers for the Thermal Shock Test (TST), which is described hereinunder.
A further object of the invention is the use of said perfluoropolyethers also for other tests the electronic circuits must commonly undergo, such as the Gross Leak Test and the Burn in Test, which too are described in the following.
2. Description of the Prior Art
The Thermal Shock Test (TST), the modalities of which are described in U.S. MIL STD 883-1105,1 consists in submitting the electronic components to high and low temperature thermal cycles, subsequently testing both the physical characteristics of the materials, and their electrical functional characteristics.
In practice, the components are alternatively and repeatedly dipped into a hot inert fluid, and into a cold inert fluid. The temperatures at which the tests are usually carried out depend on the reliability degree required to the electronic components. The most suitable temperature couples are: -55.degree. and +125.degree. C.; -65.degree. and +150.degree. C.; -65.degree. and +200.degree. C.; in these cases, temperature excursions of +10.degree. C. for the hot bathes and of -10.degree. C. for the cold bathes are allowed.
It is required that the transfer of the electronic devices from the one to the other bath and vice-versa takes place within very short times, not longer than 10 seconds.
For this test, highly fluorinated fluids are commonly used. It is known in fact that the compounds having a high fluorine content show an exceptionally favourable combination of excellent characteristics, such as the chemical inertia, the thermal stability, the non-flammability, the high electrical resistivity, the low surface tension, the poor solubility in water, the compatibility with many materials, such as elastomers, plastomers and metals.
Perfluoroalkanes having linear or cyclic structure obtained by fluorinating aliphatic, cycloaliphatic or aromatic hydrocarbons are known. An example representative of the cyclic compounds is perfluorodimethylcyclohexane, obtained by reacting xylene with CoF.sub.3.
However, the so obtained fluids are not completely fluorinated, because they contain byproducts still having hydrogen atoms. The presence of such byproducts decreases the thermal stability and the chemical inertia of such fluorinated fluids, limiting their application field.
Moreover, even when these fluids are completely fluorinated, they have not very high boiling temperatures when their pour points are very low.
By "pour point" the temperature is meant at which the liquid, upon cooling, modifies its physical characteristics, i.e., at which its flowing capability decreases, because its viscosity increase. In general, as the pour point the temperature is considered at which the viscosity reaches the value of 100,000 cSt (ASTM D 97 Standard).
For example, for pour points of -70.degree. C. the boiling temperature is at the most of the order of 100.degree. C., whilst the products having higher boiling temperature, of the order of 210.degree. C., suffer from the disadvantage of having too high pour points, of the order of -20.degree. C.
Also perfluorinated compounds having ether or aminic structure, obtained by electrofluorination in hydrofluoric acid of the corresponding hydrogenated compound are known. An example representative of these compounds is perfluorotributylamine.
In this case too, the fluids which are obtained are not completely fluorinated, as it occurs for the above described fluids, and show the same disadvantages.
In Table 1, the physical characteristics of some of the hereinabove mentioned products are reported.
TABLE 1 ______________________________________ Boiling point Pour point Product T .degree.C. T .degree. C. ______________________________________ Perfluorotributylamine 174 -50 Perfluorotripentylamine 215 -25 Mixture of cyclic ethers of 97 -110 formula C.sub.8 F.sub.16 O Perfluorodimethylcyclohexane 102 -70 ______________________________________
It can be seen that a group of these products can be used only at high temperatures, whilst another group can be used only at low temperatures. In fact, the range from the boiling temperature and the pourpoint is generally relatively wide, but not wide enough to enable the same perfluorinated compound to be used both at high and low temperatures.
The compounds belonging to the class of the amines, when have rather high boiling points, have high pour points; the class of cyclic ethers has pour point which can be also very low, but the respective boiling temperatures are low.
The use of couples of different fluids for the low and the high temperatures in the TST implies problem of both practical and economic character.
In practice, the rapid transfer of the pieces being tested from the cold to the hot bath and vice-versa, because of the dragging of the fluids, causes: (1) mutual pollution of the baths, with consequent change of the physical-chemical characteristics of the fluids; (2) loss by evaporation of aliquots, which can also be substantial, of the low-boiling fluid, when this is dragged into the high-temperature tank and when the hot pieces are dipped into the cold bath; (3) increase of the viscosity of the low-boiling fluid polluted by the high-boiling fluid; (4) contemporaneous lowering of the level of the cold fluid, and increase of that of the hot fluid (actually, it occurs a greater dragging of the cold fluid, which is very viscous under the use conditions); (5) need for the equipment to be stopped from time to time to replace the fluids; (6) need for a rectifier unit to be available for the reclaiming of the two mutually polluted fluids.
The demand hence existed for a fluid suitable to be individually used both at high and low temperatures, so as to avoid the above described drawbacks, and such to be used also for other tests of electronic industry, such as Gross Leak Test and Burn in Test.